Floor pan removal and replacement?

To brace the flooring, run some angle bar in an X pattern between the door sills, tack welding it to the body. Maybe even an X for the front and one for the rear. This will prevent the body from twisting and sagging as you cut the floor pan out. Also add some stands under the floor area, not just on the front and rear frame rails. If you don't, you may get the new floor welded in and it looks great, then the doors don't close correctly an the car "crabs" down the road. Also, when welding you never weld in a solid bead on sheet metal. Make sure you weld the perimeter in short, spaced out sections, filling in the open areas in 5-6 trips around the perimeter.